1. Field of the Invention
The present invention relates to a motor drive to drive a motor, and specifically relates to a motor drive that detects a circuit abnormality owing to foreign matter having entered the motor drive, before a significant abnormality occurs in the motor drive.
2. Description of Related Art
Motor drives have the problem of foreign matter such as cutting oil entering the inside of the motor drives. The entering foreign matter may affect the characteristics of the motor drives and cause operational malfunctions. Particularly when miniaturized high resistance components are used, the entering foreign matter causes leakage current and lowers the resistance values of the components to less than their original values, thus deteriorating the circuit characteristics. To detect the foreign matter, a detection circuit is required to be separately provided.
This problem will be described in more detail. The motor drives contain various detection circuits. In particular, the motor drives need to measure a high voltage. Thus, resistors are used to divide the voltage to be detected. As resistors for dividing the voltage, resistors having a high resistance value are used in order to reduce current consumption and a loss of current flowing through the resistors during measurement.
Taking a high voltage measurement circuit 1000 shown in FIG. 1 as an example, a voltage measurement circuit 1003 measures a divided voltage. A voltage measurement result by the voltage measurement circuit 1003 is multiplied by a division ratio to calculate a high voltage value to be obtained. For example, when R1 represents the resistance value of a resistor 1001, R2 represents the resistance value of a resistor 1002, and V represents a measurement value of the voltage measurement circuit 1003, the voltage V0 between two terminals 1004 and 1005, that is, a voltage to be measured is calculated by the following expression:V0=(R1+R2)/R2×V 
However, as the division ratio, there is no choice but to use a numerical value based on a design value. As a result, in the event that entering foreign matter alters the resistance value of the resistor 1001 or 1002 for dividing the voltage, a voltage measurement result after voltage division becomes abnormal, thus losing the ability to measure a normal voltage value. The entering foreign matter may form a path for leakage current that alters the characteristics of the high resistance component so as to have a lower resistance value than its original value, or may cause a wire break by corrosion.
As a device for detecting a circuit abnormality, a load driving device in which a plurality of detectors for detecting a voltage or a current are provided, and when at least one detection value exceeds a predetermined allowance, a broken portion is identified and operation is continued if possible is known (for example, Japanese Unexamined Patent Publication (Kokai) No. 2007-252134). The conventional art is based on the premise that the single detector by itself makes a determination of normal or abnormal. If the single detector cannot make a determination of normal or abnormal, an abnormality cannot be detected.